\documentclass[12pt]{article}
\usepackage{geometry} 
\geometry{letterpaper}
\usepackage{graphicx}
\usepackage{amssymb}
\usepackage{fontspec,xltxtra,xunicode}
\setmainfont[Mapping=tex-text]{宋体}
\defaultfontfeatures{Mapping=tex-text}
\setromanfont[Mapping=tex-text]{宋体}
\setsansfont[Scale=MatchLowercase,Mapping=tex-text]{宋体}
\setmonofont[Scale=MatchLowercase]{宋体}

\begin{document} 

\newfontfamily{\K}[Scale=1.9]{Kai}
\newfontfamily{\F}[Scale=1.9]{FangSong}
\newfontfamily{\H}[Scale=1.85]{SimHei}


\HUGE 我的演示文稿是正确的!精彩的!\\

\HUGE {\K 楷体}\\

\HUGE {\F 仿宋体}\\

\HUGE {\H 黑体}\\

\end{document}